Battery on the way out?

[wasabi]

So, new to me Audi A3 1.8T, although actually ancient 1998 vehicle.

Starts okay - no issues there at all.

Put FLI 12 inch active sub in the boot today and connected up new Pioneer head unit. Head unit is working okay with regular car speakers powered by internal amp.

The issue i'm seeing is that the sub isn't running but seems to be getting power. With key removed from ignition / engine off, i can measure 12.7v at the + input on the sub. However as soon as i switch on stereo the readings drop to between 3.5 and 5.1v, no sound from sub although head unit driving door speakers still fine. Same reading on the remote on/off skinny power cable. this is measuring with - on the sub at the point where it leaves to go off to bolt in the boot.

So far tested as follows:

Was originally using slighter narrower power cable already installed in vehlcle but later used my own cable that came as part of installation kit and no change to readings. i.e. don't think cable guage an issue as have used this cable for 18 months without issue.

Have moved the - cable to various points on chassis. This did help a fraction but not the back to 12v i'd hoped for.

Not tested trying to work with engine running, or testing alternator - only thought of that just now. However keep coming back to -the car starts fine-. Nor have I been able to find any old 3.5 mm jack ot phone cables to send audio signal to sub direct - they're all hiding in the junk cupboards somewhere.


My questions are this:

Am i missing something forehead-slappingly obvious regarding the voltage? i.e. is there any sort of behavior where an amp pulls variable levels of voltage? i can't see how it would - surely voltage constant and amps change, which leads to question 2

How do i measure the max ampage a battery is capable of delivering? The car starts, lights work etc, but it is a 1 kw amp and the battery is quite small....

Basically looking to be certain battery at fault before i lay out money for no good reason.

[peterb]

The amp will take considerably less current than the starter motor, so if the car is starting OK, the fault lies in the amp or, more likely, the wiring to it. Measure the voltage at the battery when the ignition is turned on - my guess is that it will not drop at all. The voltage drop is occurring on the power cables to the sub amp; a dodgy connection at one end of the power or earth return lead. (I am assuming you are using a negative return lead, and not using the car chassis.)

A 1KW amp at 12 volts would take about 80 amps if it was a DC load. but it isn't, and your 1KW amp will probably be 1KW peak value if you read the small print - the RMS output is likely to be <250 Watts - which equates to a more realistic 20A load. (The starter motor will take several hundred amps when it cranks a cold engine - which is why the cables are so thick).

You cannot easily measure the max current the battery will supply - but a good battery will have an instantaneous short circuit current of many hundreds of amps - which is why shorting a car battery is so dangerous. In principal though, a high current calibrated load to the battery and measure the voltage drop at the terminals when it is connected. That wil give an idea of the internal resistance of the battery. DO NOT TRY THIS YOURSELF!

You need to check the integrity of the connections of the connecting cables - somewhere you will have a high resistance joint.

Just re-read your post - you are using a bolt in the boot as the negative/ground point. If I was a betting man, I'd say that is likely to be the problem point. Run a negative lead from the sub amp directly back to the battery.

[wasabi]

The amp will take considerably less current than the starter motor, so if the car is starting OK, the fault lies in the amp or, more likely, the wiring to it. Measure the voltage at the battery when the ignition is turned on - my guess is that it will not drop at all. The voltage drop is occurring on the power cables to the sub amp; a dodgy connection at one end of the power or earth return lead. (I am assuming you are using a negative return lead, and not using the car chassis.)

A 1KW amp at 12 volts would take about 80 amps if it was a DC load. but it isn't, and your 1KW amp will probably be 1KW peak value if you read the small print - the RMS output is likely to be <250 Watts - which equates to a more realistic 20A load. (The starter motor will take several hundred amps when it cranks a cold engine - which is why the cables are so thick).

You cannot easily measure the max current the battery will supply - but a good battery will have an instantaneous short circuit current of many hundreds of amps - which is why shorting a car battery is so dangerous. In principal though, a high current calibrated load to the battery and measure the voltage drop at the terminals when it is connected. That wil give an idea of the internal resistance of the battery. DO NOT TRY THIS YOURSELF!

You need to check the integrity of the connections of the connecting cables - somewhere you will have a high resistance joint.

Just re-read your post - you are using a bolt in the boot as the negative/ground point. If I was a betting man, I'd say that is likely to be the problem point. Run a negative lead from the sub amp directly back to the battery.

Thanks. Rereading my own post now several hours later, the fact that the remote switch cable is also showing the same voltage would make me suspect negative wire too. Shopping trip time.

Onboard fuse on the sub is rated 20A so suspect you're right in the RMS estimate, although I use an extra 40A inline fuse just after the battery too.

[GoNz0]

Back in the day (When I was a lad!) We never used the ground to chassis method as it never worked well with amps.

[peterb]

Thanks. Rereading my own post now several hours later, the fact that the remote switch cable is also showing the same voltage would make me suspect negative wire too. Shopping trip time.

Onboard fuse on the sub is rated 20A so suspect you're right in the RMS estimate, although I use an extra 40A inline fuse just after the battery too.

Yes. The 40A fuse close to the battery is a good idea as it protects the connecting cable should it be damaged. From an audiophile point of view, you need to use thick cables to the sub amp as on loud bass notes/drum beats, the amp will be drawing current pulses. A thin cable will exhibit a voltage drop during those pulses and that will affect the amplifier's performance - although in practice in a car, it probably wouldn't be that audible!

Anyway, hope you get it all sorted!

Back in the day (When I was a lad!) We never used the ground to chassis method as it never worked well with amps.

QFT!! Always run back a separate ground cable! On modern cars with CAN-BUS systems, I think the main power bus has a negative cable and ground return is less common. That certainly seems to be the case where the battery is installed outside the engine compartment and a substantial connection is required for the starter motor.

[wasabi]

All worked out okay with dedicated ground cable. Sounds superb - i know people take the mick out of big subs in boots but it fills out the sound superbly without being boomy.

On previous car going straight to chassis was okay, but I should have learned my lesson from years of wonky 90s motorbike electrics where they used chassis-grounds, badly. Endless bending things to try to get them to make a connection - or wedging bits of aluminium foil in as emergency fixes for indicator failures.

Downside being I lost much of today doing cable runs when i really should have being doing the brake pads showing as an advisory on last month's MOT. ... next weekend, if i don't get sidetracked fitting a towbar....

[GoNz0]

well at least while you're sat upside down in a ditch you will have a banging bass line

[peterb]

Glad you got it all sorted! Enjoy!